Typical bill authentication devices which utilize the magnetic hysteresis properties of the material of a secured document—such as currency—employ at least one static magnetic field. Other bill authentication devices employ two static magnetic fields of the same or opposite polarities. The use of two magnetic fields allows for a measure of both the saturation magnetization and the non-saturated magnetization. Where fields of opposite polarities are employed, the choice of the reverse polarity field is such that not only the magnitude of the output is changed, but the polarity (phase) is also changed. In typical bill authentication devices, permanent magnets are used to create one or two static magnetic fields.